Radioactive measuring devices include radioactive radiators, which, during operation, send out radioactive radiation through the container, and include detectors, which serve to detect a radiation intensity penetrating through the container, which is dependent on the physical, measured variable to be measured, and to convert this into an electrical output signal. Radiometric measuring devices are typically always applied when conventional measuring devices are not applicable due to especially rough conditions at the measuring location. Very frequently, extremely high temperatures and pressures reign, for example, at the measuring location, or highly chemically and/or mechanically aggressive environmental influences are present, which make the use of other measuring methods impossible.
In radiometric measurements technology, a radioactive radiator, e.g. a Co 60 or Cs 137 preparation, is installed in a radiation protection container, and placed at a measuring location, e.g. a container filled with a fill substance. Such a container can be, for example, a tank, a pipe, a conveyor belt, or any other form of container.
The radiation protection container has a window, through which the radiation emitted by the radiator positioned for measuring is radiated through a wall of the radiation protection container.
Usually, a radiation direction is selected, in the case of which the radiation penetrates that region of the container, which should be metrologically registered. On the oppositely lying side, the radiation intensity emerging from the container over a region to be metrologically registered (this intensity being dependent on the fill level or on the density of the fill substance) is quantitatively registered with a detector. The emerging radiation intensity depends on geometric arrangement and absorption. The latter of these is, in the case of fill level measurement and in the case of monitoring an exceeding or subceeding of a predetermined fill level, dependent on the amount of fill substance in the container, and in the case of density measurement, on the density of the fill substance. As a result, the emerging radiation intensity is a measure for the current fill level, the superseding or subceeding of the predetermined fill level, or the current density of the fill substance in the container.
Today, usually scintillation detectors having a scintillator, e.g. a scintillation rod, and a light receiver, e.g. a photomultiplier, are used as detector. The scintillation rod is composed of a special plastic, e.g. polystyrene (PS) or polyvinyl toluene (PVT), which is very optically pure. Gamma radiation triggers light flashes in the scintillation material, whose light is registered by the photomultiplier and converted into electrical pulses. Connected to the photomultiplier is a measuring device electronics, which, based on the electrical pulses, determines a pulse rate with which the pulses occur. The pulse rate is dependent on the radiation intensity, and is thus a measure for the physical variable to be measured.
Solid scintillation rods have, however, the disadvantage that, due to their dimensions, they cannot at all or can only very poorly be connected to light receivers which today are obtainable in very small forms of construction, since, in such case, a large part of the light would radiate unused past the light receiver. Correspondingly, such scintillation rods are usually used in connection with large and expensive photomultipliers.
Added to this is the fact that, in the case of solid scintillation rods, due to manufacturing-related surface defects, a portion of the light escapes laterally from the rod, and is therewith lost for metrological registration.
Detectors are known, in the case of which, instead of solid scintillation rods, scintillating fibers are applied. Scintillation fibers have, as a rule, a diameter in the order of magnitude of 1 mm, or in the case of fibers with a polygonal cross section, a cross sectional area in the order of magnitude of 1 mm2, and can accordingly be connected very well to small light receivers.
In JP 09 080156 A, a radiometric measuring arrangement is described, which serves to measure a radiation dose emerging from a radioactive fill substance located in a container. For this, a detector is used, which has a helical scintillation fiber wound around the container, on whose two ends a light receiver, here a photomultiplier or an avalanche photo diode, is, in each case, connected. Radiometric radiation emerging from the fill substance produces light flashes at locations along the fiber impinged upon by the radiation, with these light flashes propagating toward the two ends of the scintillation fiber. Connected to the two light receivers is a signal processing unit, which determines a travel time difference of the received signals attributable to one and the same light flash, and, based on the propagation velocity of the light signals in the fiber, determines therefrom the location of origination of the associated light flash.
This arrangement is, however, in the described form only useable in connection with radioactive fill substances, since the fiber surrounds the container on all sides. In connection with the above named measuring arrangement, in the case of which a radiation source arranged outside of the container is used, this arrangement would essentially metrologically register the radiative power of the source. Moreover, the length of the scintillation fiber is limited, since the light is attenuated in the fiber. Correspondingly, the arrangement is only useable in connection with relatively small containers.
In comparison to a solid scintillation rod, an individual scintillation fiber has the disadvantage that it has a considerably smaller irradiated mass. Accordingly, the radiative power that impinges on an individual scintillation fiber is, in comparison, very small.
This low irradiated mass can, for example, be compensated by the measuring arrangement described in EP 1 068 494 B1, wherein a detector is used, in which a number of scintillation fibers are combined to form a bundle, whose diameter is greater than the diameter of the individual fibers. The entire bundle is connected at one end to a photomultiplier, which converts the light conveyed over the scintillation fibers into an electrical signal.
Due to an irradiated mass of in the fiber bundle which is increased in comparison to an individual fiber, the radiative power received by the detector is increased. However, a large, expensive photomultiplier is still made use of here. Due to the attenuation of the scintillation light in the fibers, the length of the bundle is limited. Moreover, fiber bundles are relatively rigid and inflexible. Through this, the region metrologically registerable with the scintillation fiber bundle, is constrained.